One-way clutches used e.g. in paper feeders of printers, copiers, facsimile machines, etc. include spring type clutches, which include a clutch spring adapted to be selectively pressed against and released from a driving or driven member, thereby selectively transmitting torque.
Spring type one-way clutches include those having a clutch spring in the form of a single-layered spring, as disclosed in “Machine element utilization manual-Spring”, edited by Nippatsu-NHK Spring Co., Ltd, published Feb. 15, 1995, page 136, and JP Patent Publication 63-6229A, and those having a double-layered clutch spring, as disclosed in JP Patent Publication 2008-101740A.
A single-layered spring type one-way clutch includes a driving shaft (driving arbor), a driven shaft (driven arbor) coaxial with the driving shaft, and a spring fitted over the end portions of both shafts such that when the driving shaft is rotated in one direction, the spring is radially compressed and pressed against both shafts, thereby transmitting the rotation of the driving shaft in the one direction to the driven shaft. One drawback of this type of clutches is their small torque transmission capacity. If it is desired to transmit larger torque, it is necessary to use a clutch spring with a large number of turns, which increases the size of the one-way clutch.
A double-layered spring type one-way clutch includes a double-layered clutch spring including a large-diameter coil spring portion kept in elastic contact with a clutch surface formed on the inner periphery of the driving ring, and a small-diameter coil spring portion kept in elastic contact with a clutch surface formed on the outer periphery of the driven ring such that when the driving ring is rotated in one direction, the large-diameter coil spring portion is radially expanded while the small-diameter coil spring portion is radially compressed, whereby the large-diameter coil spring portion and the small-diameter coil spring portion are pressed against the respective clutch surfaces of the driving ring and the driven ring. The driving ring is thus coupled to the driven ring and the torque of the driving ring in the above-mentioned one direction is transmitted to the driven ring. This one-way clutch is thus large in torque transmission capacity while its axial length is small.
While the double-layered spring type one-way clutch disclosed in JP Patent Publication 2008-101740A is short in axial length, since the large-diameter coil spring portion and the small-diameter coil spring portion are connected together through a U-shaped transition portion, i.e. a portion bent by 180°, a relatively large stress tends to be repeatedly applied to the small bent portion of the transition portion which is bent with a small radius of curvature every time rotation is transmitted from the driving ring to the driven ring. Stress thus tends to accumulate in the transition portion, increasing the possibility of damage to the transition portion due to fatigue. The durability of such a clutch spring is therefore not sufficient.